The present invention relates generally to transimpedance amplifiers, and more particularly, to differential transimpedance amplifiers with automatic gain control.
Receivers in optical communication systems use transimpedance amplifiers to convert current outputs of photodetectors to voltages. In these applications, average detected power can vary by two or three orders of magnitude depending upon the particular optical fiber link used to transmit the signal. Accordingly, the transimpedance amplifier must be designed to operate over a wide range of input currents. This is sometimes accomplished by automatically controlling the gain of the transimpedance amplifier.
A conventional single-ended transimpedance amplifier with automatic gain control (AGC) is shown in FIG. 1. A photodetector 10 is used to convert an optical input into a current. For analytical purposes, the photodetector is modeled by an ideal current source 12 in parallel with a capacitor 14. The photodetector 10 is connected to the input of an inverting amplifier 16 with an open-loop gain of (-A). A feedback circuit comprising a feedback resistor 18 connected in parallel with a feedback field effect transistor 20 (FET) is coupled between the input and output of the amplifier 16. The output of the amplifier 16 is connected to an AGC circuit 22. An AGC buffer 21 in conjunction with a shunt capacitor 24 produces an AGC voltage at the output of the AGC buffer. The AGC voltage is the average voltage of the amplifier 16 output. The AGC voltage is applied to the gate of the feedback FET 20.
In operation, a voltage is developed at the output of the amplifier 16 due to the flow of current in the feedback circuit. The average output voltage of the amplifier 16 is applied to the gate of the feedback FET 20 via the AGC buffer 21 in conjunction with the shunt capacitor 24. For low optical input power the average output voltage of the amplifier 16 is low, the feedback FET 20 is off, and the effective transimpedance of the amplifier 16 is equal to the value of the feedback resistor 18. As the optical input power increases, however, the average output voltage of the amplifier 16 increases, causing the feedback FET 18 to turn on. This reduces the effective impedance of the feedback resistor, and thus the transimpedance of the amplifier 16, and thereby maintains the amplifier 16 in the linear region. Thus, the AGC circuit 22 ensures that the amplifier 16 is not overloaded. In the absence of the AGC circuit 22, large voltages developed across the feedback resistor 18 may cause severe distortion and jitter in the output of the amplifier 16.
A common problem encountered with such a conventional transimpedance amplifier employing AGC is decreased stability. In the closed-loop system described in FIG. 1, for example, the frequency of the dominant pole (P1) is set by the feedback resistor 18 (R.sub.f), the total input capacitance (C.sub.in), and the open-loop gain of the transimpedance amplifier 16 (-A) according to the formula P1=-A/2(R.sub.f C.sub.in) radians/sec., where C.sub.in is the sum of the photodetector capacitance 14, the input capacitance of the transimpedance amplifier 16 and any parasitic capacitances. As long as P1 is sufficiently far from the pole of the transimpedance amplifier 32 as to have a phase margin of 45.degree., stability is maintained. When the feedback FET 40 turns on, however, R.sub.f is effectively reduced, thereby increasing P1. As P1 moves towards the frequency of the transimpedance amplifier pole, which is generally at a higher frequency, the phase margin is reduced. To maintain good phase margin, the open-loop gain (-A) of the transimpedance amplifier 32 must be reduced. The reduction in open-loop gain requires additional circuitry not shown. Accordingly, there is a current need for a transimpedance amplifier with AGC that maintains stability with minimal additional circuitry.